Breed-and-burn, traveling wave reactors as a new medium for harnessing nuclear energy



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Session B9

Paper #4186

BREED-AND-BURN, TRAVELING WAVE REACTORS AS A NEW MEDIUM FOR HARNESSING NUCLEAR ENERGY
James Bumstead (Jrb193@pitt.edu, Budny 4:00), Sean McCarthy (Stm76@pitt.edu, Budny 4:00)



Abstract—With the world’s current dependence on finite fossil fuels such as coal and oil, engineers have diligently been searching for safe, efficient and clean alternative energies. One promising alternative is the utilization of nuclear power. However, currently used light water reactors (LWRs) have received some criticism on their efficiency and amount of waste production. As a response, TerraPower, a nuclear energy technology company, is focusing its efforts on developing a new system for producing nuclear energy: the Traveling Wave Reactor (TWR).

In this paper, we will show how the TWR could potentially increase the fuel consumption and efficiency seen in traditional LWRs through the use of more naturally abundant fuel. The elimination of fuel enrichment plants will result in a reduction of operation and transportation costs, and will reduce the risk of nuclear proliferation. By switching from water to liquid sodium as a primary coolant, TWRs will benefit from reduced neutron moderation and more effective heat exchanges.

An analysis of recent TWR designs will be provided, focusing on their economic, safety, and environmental impacts. Before discussing the advanced techniques of TWRs, we will provide a basic, foundational outline for the process of nuclear fission and power production. For a point of reference, TWRs will be compared to the currently implemented LWRs.
Key Words—Breeder Reactor, Fast-Reactor, Mechanical, Nuclear Energy, TerraPower, Traveling Wave Reactor.
NUCLEAR ENERGY BASICS AND HISTORY
As new technologies lead to a globalizing trend of modernization, one of the greatest problems becomes how to sustain this growth with the energy resources that are available on Earth. In 2012, the United States received about 81% of its energy from finite fossil fuel resources according to the Energy Information Administration [1]. Problems with this heavy dependence on fossil fuels are the resulting carbon dioxide pollution and the inherent lack of long-term reliability. According to peak oil theory, oil production is predicted to reach a maximum rate by as soon as 2020 and then decline indefinitely [2]. With all nonrenewable resources having this property of impermanence, scientists have made it a priority to find new methods of energy production. In looking for solutions to this dilemma, the sustainability of all potential solutions is a very important factor. Nuclear energy offers such solutions in terms of quality of life assurance and environmental protection. While not a renewable resource, the fuel used in nuclear reactors is very abundant on Earth, and if used effectively, it could aid in sustaining the world’s energy demands.

It was not until after the end of the Second World War and the race for nuclear weaponry that nuclear technology was investigated with the intent of creating an alternative means of power production. In 1960, Westinghouse built the first commercial pressurized light water nuclear reactor, utilizing the concepts of fission that were once fundamental to the creation of the atomic bomb [3].

Fission is the splitting of a heavy nucleus into smaller fragments as a result of the absorption of a free neutron [4]. For example, one of the common reactions that take place in reactors today is as follows:

When calculating the masses involved in such a reaction, it can be seen that the mass of the products is slightly smaller than the mass of the reactants, with the difference being released as kinetic energy according to the equation E=mc2, which accounts for the breaking of strong nuclear forces [4]. The fact that large energies can be produced from small mass differences is what makes nuclear technology such an attractive prospect.


LIGHT WATER REACTOR BASICS
There are currently 104 light water reactors (LWRs) operating in the United States, 74 of which have been operating for more than 60 years [5]. Like most nuclear reactors, LWRs run on the fissioning of uranium, a common element found in the earth’s crust. Per year, the world’s reactors require about 68,000 metric tons of uranium to function [6]. However, for uranium to be used in a reactor, there must be a very thorough enrichment process. This will be further discussed in later sections.

Together, all of these LWRs produce over 100 GW of the United States’ electrical capacity which is about 20% [5]. While these statistics are impressive, the public remains skeptical about nuclear power. One concern is the high radioactivity of uranium. To be radioactive means to be highly unstable, and unless properly handled, radioactive elements can have negative effects on health and the environment. New reactor designs, however, are attempting to address these concerns even more directly than current LWRs.


Light Water Reactor Systems and Processes
LWRs follow the same processes of a generic nuclear reactor in that through nuclear fission, heat is generated. The nuclear reaction occurs within a fuel rod, which is placed in a large vessel filled with water. The water is then heated to a boil and converted into steam. The steam leaves the central core through steam valves and then powers a generator, thus producing electricity. To expedite the process, many rods are placed within the core, allowing for a continuous chain of nuclear fission to occur. When a single neutron collides with the first bit of uranium, several more neutrons are released. The newly released neutrons collide with other fissile materials and create a fast acting chain reaction [4].

What characterizes LWRs is their use of natural water as a coolant and moderator. Neutrons from fission are released at extremely high speeds and kinetic energies of around 5 megaelectron-volts (MeV) [4]. To reduce these energies, engineers use a moderator; in the LWR’s case, water is used. By reducing the kinetic energies, there is a far greater probability of the released neutrons colliding with other fissile materials.


Types of Light Water Reactors and Their Fuel
In actuality, there are many different subcategories of LWRs. The United States generally uses the pressurized water reactor (PWR) and the boiling water reactor (BWR). These two reactors follow the same general description as mentioned before, but there are a few key differences between them.

For one, the PWR’s reactor core is pressurized, which gives the reactor its name. The reactor core is pressurized to around 160 atmospheres (atm), allowing the water passing through as a moderator to not boil until 315°C [7]. Therefore, to generate steam, water is pumped out of the core through the primary loop to heat water within a secondary, segregated, less pressurized loop. There, water within the secondary loop is converted to steam to power a generator. This can be clearly seen in Figure 1. Given the event of a fuel cladding failure within the core, fission products will not be passed to the turbine as steam. Though PWRs are more expensive, they are the more efficient reactor of the two types of LWR.

On the other hand, BWRs follow a simpler, cheaper core design. In this case, the reactor core is only slightly pressurized, around 70 atm, making the water boil at 285°C. What distinguishes BWRs from PWRs is that there is no segregation between loops, as is shown by Figure 2. Water reaching a boil within the reactor core serves as the source of steam for the turbine. To replenish the core, water is fed in through the feedwater pump from an outside source. Unlike with PWRs, in the case of a fuel cladding failure, radioactive fission products may escape and be passed through the turbine. This can prove to be highly dangerous and can lead to environmental harm. Though BWRs are a simpler and cheaper reactor, they are not as common in the United States as PWRs because they are less efficient and not as safe [7].
FIGURE 1 [7]

The segregation between chambers for PWRs
PWRs and BWRs both run on uranium, or more specifically, U-235. U-235 is a fissile isotope of uranium. An isotope shares the same amount of electrons and protons as its natural element, however it differs in the amount of neutrons it holds. Unfortunately, natural uranium has very little U-235 per mass (0.7%), meaning very little fuel can actually be utilized for energy [6]. To account for this, there are facilities called enrichment plants. Through a series of chemical processes, U-235 is enriched to nuclear grade or about 4% U-235 [6]. However, TerraPower, a nuclear energy company, is looking to eliminate this process through the use of a new model of reactor.
FIGURE 2 [7]

BWRs, though cheaper, have no segregation between chambers, which could lead to water contamination by escaped fission products
TERRAPOWER AND THE TRAVELING WAVE REACTOR
Inspired by experimental breeder reactor designs of the past such as the CANDLE reactor in Tokyo, TerraPower was formed from Intellectual Ventures in 2008 under the head of John Gilleland [8]. Initially aimed at creating a reactor which uses a traveling wave to breed and burn fuel, it later adopted the idea of a standing wave through which fuel is moved in 2011. The advantages which were advertised for the technology were so exciting that billionaire Bill Gates began funding TerraPower in his search for new sources of carbon-free energy.

The Traveling Wave Reactor (TWR) offers many solutions to the problems with sustainability of past reactor models. In terms of quality of life assurance, the TWR offers a new source of fuel which can produce more energy throughout its life. By using a new fuel source, the widespread use of enrichment plants will be unnecessary, thus eliminating the single source of carbon air pollution in the nuclear fuel cycle as well as reducing the risk of nuclear proliferation. Its fuel burning process also offers a reduction in the amount of radioactive waste which is produced by traditional LWRs. All of these improvements could reduce strain on the environment and could help sustain humanity’s current quality of life.


Using Naturally Abundant Uranium as Fuel
One of the most significant claims that TerraPower makes about its new reactor design is its ability to derive most of its power directly from the fissioning of U-238, an isotope which by far makes up the largest percent mass in uranium samples. By using a fast neutron reactor model along with a new approach to the burning of fuel, the non-fissile U-238 can, under the right conditions, be used to sustain a reaction in which fissile plutonium is bred and then fissioned to produce energy [1]. The full breeding reaction including intermediate decays is shown by the following equation:
[2]
By looking at this equation, it is apparent that two separate neutron interactions are happening within this process. First, a “fast” neutron with at least 0.9 MeV of kinetic energy is absorbed into a U-238 nucleus, making it unstable [4]. After two relatively quick beta decays (with half-lives of about 23.5 minutes and 2.35 days respectively) from unstable isotopes of U-239 and neptonium-239, a second neutron interacts with the fissile plutonium nucleus, breaking it into fission fragments and releasing large amounts of kinetic energy [7]. During the beta decays, a neutron in the unstable nucleus is transformed into an energetic electron and a nucleic proton [4]. When the plutonium fissions into smaller nuclides, the total kinetic energy of these fragments is on average about 166 MeV [4]. Therefore, it can be calculated that if one gram of plutonium is fissioned per day at a nuclear power plant, it will produce 0.775969 megawatts (MW) per day, which is about equivalent to the burning of 466 gallons of fuel oil [9]. This is not including the energy from neutrino production or gamma, beta, and neutron radiation which give a total of 200 MeV [4]. Nuclear fuels are said to have high energy densities because of the large amount of energy that can be produced from a small amount of mass and therefore can play a major role in resource conservation.

The most apparent advantage of using the U-238 as the primary neutron target lies in the fact that this will lead to more uranium being utilized in reactors. Instead of using the naturally scarce U-235 isotope for reactions, the 99.3% abundant U-238 will lead to an overall increase in the amount of energy available from nuclear power, a fact that will be crucial in lessening the world’s dependence on carbon-based fossil fuels. It is important to note that U-238 is not only available in natural samples, but it is also a byproduct of reactions in traditional LWRs. Thus what was once thought of as waste (depleted uranium) could potentially be reprocessed for use in TWRs. Though the United States does not currently reprocess spent fuel, changes in regulations could result in a substantial increase in the amount of fuel available. TerraPower claims that “A TWR fleet could meet the U.S. residential sector's energy needs for more than 700 years, just using the current U.S. stockpile of depleted uranium” [4].


Neutron Energies and Fast Reactors
In LWRs, low energy or “thermal” neutrons are used. This is due to the higher probability of neutron absorption (leading to fission) by a target nucleus (which is uranium-235 in the case of LWRs) when slow neutrons are used [6]. This probability can be quantified by the nuclear cross section σ, which can be thought of as the characteristic area in which neutron interactions can take place and is measured in barns (b), where one barn is equivalent to 10-28 m2 [6].

In attempting to increase the amount of fuel being consumed, it is necessary to use fuel with high neutron cross sections to maximize the amount of interaction between the fuel and the free neutrons. Neutron cross sections for all isotopes can be related to the speed and kinetic energy of incident neutrons. Relationships for fission cross sections for U-235, U-238 and Pu-239 given incident neutron energies is shown in Figure 3.


Figure 3 [6]

Fission cross sections for U-235, Pu-239, and U-238 for thermal and fast neutrons

What can be seen from the graph is that the fission cross sections of U-235 are much greater in the thermal neutron region than in the fast neutron region. Conversely, fissioning of U-238 is extremely improbable from thermal neutrons. For these reasons, U-235 is traditionally used as the main fission component in LWRs, using water as a means of reducing neutron energies.

A fast reactor, however, uses fast neutrons, which are absorbed by a U-238 atom according to its corresponding absorption, or neutron capture, cross section, which is shown in Figure 4. This is what leads to the production of fissile plutonium. It can also be seen in Figure 3 that U-238 has the interesting property of having higher fission cross sections at higher neutron energies, a key feature which helps to sustain a chain reaction by supplying more fast neutrons.
Figure 4 [10]

Neutron capture cross sections for U-238 at high incident neutron energies (.05 MeV to 10 MeV)
These characteristics of U-238 are critical for TWR functioning since the main reaction involves neutron absorption which in turn breeds plutonium-239 as fuel. In this case no neutron moderator like water is employed since high neutrons energies need to be maintained. The plutonium is then fissioned by neutrons in a “burn” wave, releasing similar amounts of energy as in a U-235 fission in a LWR. Thus, each fission provides about the same amount of energy while 99.3% of the uranium sample is being targeted for burning in a TWR compared to the only 4% of U-235 targeted in enriched fuel for LWRs [6].
Uranium Enrichment Process
Another advantage of switching to a U-238 fuel supply is that enrichment plants will no longer be a necessary part in the fuel cycle. One of the most common processes of uranium enrichment is gaseous diffusion. In this case, uranium is converted to uranium hexafluoride (UF6) or “hex” [4]. Enrichment occurs within a cylinder with two halves separated by a porous membrane. From a feed at one end, hex is pumped into one half. Due to the change in pressure, hex diffuses across the membrane. By Graham’s law, the ratio of the two diffusion speeds are inversely proportional to the square root of their relative masses [3]. Since U-238 is heavier, it diffuses more slowly, and sometimes cannot pass through while the faster U-235 can. However, since their masses only differ by about 3 atomic mass units, the amount of enrichment per cycle is extremely small, meaning this process needs to be repeated many times. Also, this process produces waste in the form of depleted UF6, which has a negligible amount of U-235. Though this depleted uranium is usually placed in storage, it could potentially be used as TWR fuel since it is mostly composed of U-238.

To produce exactly one kilogram of enriched uranium, 6.57 kilograms of natural uranium is required; this is accompanied with 5.57 kilograms of waste [3]. That is roughly 57,650 metric tons of waste produced per year to function the world’s reactors. This makes TWRs extremely advantageous on an environmental standpoint since they require very little enrichment for their fuel. Also, enrichment is the only part of the current nuclear fuel cycle which contributes considerably to carbon dioxide emissions.

Another negative aspect of uranium enrichment plants is their risk for nuclear proliferation or, in other words, the creation of nuclear weapons. Nuclear weapons are only functional with weapons grade uranium, or uranium that is 90% enriched [6]. A common fear about nuclear energy is that countries will produce weapons grade uranium since relatively little extra energy is required to make the jump from reactor grade to weapons grade. By producing power with TWRs, this fear is minimized by allowing our nuclear plants to function with—in the words of Charles Forsberg of MIT—only “one enrichment plant per planet” [11]. In this respect, TWRs offer improvement in quality of life by reducing the public’s fears of nuclear weaponry.

On an economic standpoint, switching to TWRs would mean removing unnecessary costs of transportation and infrastructure. Enrichment accounts for nearly half of the cost of nuclear fuel, with the other half coming from mining and transportation. By removing enrichment costs, 5% of the total electricity cost can be eliminated [6]. On top of this is the cost for constructing these plants. For instance, United States Enrichment Corporation (USEC) is currently working on building an enrichment plant in Ohio. From 2007 to 2010, almost $1.95 billion were spent on construction and another $2.8 billion is expected to be spent [7]. The elimination of these large expenditures will help make nuclear energy a more profitable venture. Overall, TWRs offer a substantial improvement in waste management, nuclear proliferation prevention, and the economy.


Core Design
While a TWR has never been built, many performance statistics have been generated based on computer simulations which predict burnup percentages, heat generation, and other important parameters that are necessary to sustain breed and burn waves within the core [12]. Using these statistics, TerraPower has made basic preliminary designs for the reactor core.

Similar to other reactors, the core consists of many thin fuel rods or pins bundled together into fuel assemblies. Two of the main assembly designs are a traditional 169 pin hexagonal configuration and a circular configuration made by removing three pins from each corner, leaving 151 pins in total [13]. Uranium metal-alloy fuel (U-10%Zr) is placed in the fuel rods which is surrounded by HT-9 ferritic-martensitic stainless steel cladding [14]. At the center of the core a very small amount of starter fuel is used, which consists of about 10% enriched uranium [12]. Fission gases can be released at the top of the rods so as to not interfere with the reaction area [15]. B4C control rods can also be inserted into the core in order to stop a reaction.

The entire core measures about 5.5m tall and 4m across its diameter and is immersed in liquid sodium, which will be discussed later [14]. The basic geometry of a single fuel assembly and core configuration is shown in Figure 5.
Figure 5 [15]

A 5.39 meter tall, 4 meter wide core (right) is composed of fuel assemblies (left), each of which contains 151 or 169 smaller fuel pins
The Standing Wave Concept
The idea of a self-sustaining, breed-and-burn wave was first proposed in 1958 by Feinberg and Kunegin at the II Genevan conference but was not considered a practical engineering solution until recently [16]. The idea behind a breeder reactor is to use neutrons to create more fissile fuel (plutonium) than it consumes. By using unenriched uranium and proper neutron balances, a breeding “blanket” of plutonium can be created and sustained along with a fissioning zone [17].

Contrary to what is often implied by its name and TerraPower’s earlier press releases, the wave sustained within the reactor does not move, but rather is a standing wave where plutonium is constantly bred. [8] [15]. The enriched uranium is fissioned initially, providing an average of 2.5 fast neutrons per fission at around 5 MeV of kinetic energy each [6] [4]. These fast neutrons are absorbed into U-238 nuclides, yielding fissile plutonium which is fissioned to produce energy as well as additional neutrons to sustain the reaction [17]. Because of the fast neutrons, U-238 can also be fissioned within this process, aiding in neutron production. As plutonium is used, new U-238 fuel is reshuffled remotely—without needing to open the core—into the breeding blanket.

The functionality of a breeder reactor depends heavily on the ratio of neutrons being created per neutrons being absorbed. The number of required neutrons can be mathematically related to the fuel burnup in the reactor, expressed in the number of fissions per initial metal atom (FIMA) [17]. From that relationship, it can be shown that breeder reactors require a minimum fuel burnup in order to operate. For the TWR, the estimated minimum burnup is 19.4% FIMA and the estimated highest possible burnup is 55% FIMA [17].

These burnups lead to fissions which cause about 500 displacements per atom (dpa) for the steel cladding which houses the fuel rods [17]. As of yet, HT-9 cladding is not known to be able to resist deformation passed 200 dpa, posing a great challenge to practical reactor implementation. Though it is most likely possible that further tests on different fuel clad designs or other material usage could yield a cladding which could accommodate 500 dpa, the amount of time and money that may need to be invested in order to yield satisfactory results is still an important factor that needs to be considered [17].

However, if the design difficulties can be overcome, the advantages of this method of fuel burning would be great. As opposed to thermal reactors which produce plutonium waste from chance U-238 interactions, the breeder reactor makes this its primary fuel source, quickly burning most of the plutonium it has created. According to TerraPower, “The TWR will produce a minimum of seven times less waste than today's light water reactors” [8]. Also, TerraPower claims that the energy output could be up to 50 times greater per gram of fuel than LWRs [8]. These assertions make the standing wave model a promising concept with high prospects in terms of improving the sustainability of past reactor models.
THE EFFECTS OF USING A SODIUM COOLANT
Around the 1950s, scientists began studying and utilizing liquid metal as a coolant in nuclear reactors. To satisfy as a potential liquid metal coolant, it is essential that the metal has non-corrosive properties, low neutron moderation, cost effectiveness, abundance, and safety. With this in mind, TerraPower selected liquid sodium as its coolant, in contrast to the LWR’s water moderator. Being 2.4% of the Earth’s crust, sodium serves as an excellent coolant for a fast reactor like a TWR [18]. Unlike the LWR, which uses water as a moderator to reduce the kinetic energy of released neutrons, the TWR employs liquid sodium in order to maintain high neutron energies. In other words, liquid sodium is ineffective as a neutron moderator. This occurs in part because sodium is much heavier compared to water, thus less kinetic energy is lost when neutrons collide with the sodium molecules. As mentioned before, U-238’s fission cross section is much higher at greater neutron energy, so it is essential for neutrons to have sufficient energy to cause fission, which aids in sustaining a change reaction.

Additionally, liquid sodium’s inherent chemical properties serve nicely for TWRs. For one, liquid sodium boils at 883°C under atmospheric pressure whereas a TWR’s core only heats to around 550°C. This means that the TWR’s core does not need to be pressurized to prevent liquid sodium from boiling, which in itself is costly [6]. Secondly, due to sodium’s relatively large specific heat, it has efficient heat transfer properties. Specific heat is the amount of heat per unit mass that is required to raise temperature by 1°C. This is essential for fast reactors like the TWR because sodium can then effectively transfer heat from the core without risk of boiling even at atmospheric pressure. While sodium possesses enticing characteristics like a high boiling point, specific heat, and the ability to not corrode steel, it also has a few dangerous, reactive qualities [19].


Sodium’s Safety Disadvantages
The greatest concern with using liquid sodium as a coolant is its highly reactive nature with water to form hydrogen gas. Also, when making contact with oxygen, sodium becomes increasingly more corrosive. Consequently, it is essential that neither come into contact with sodium, whether it be inside of the reactor nor in the transportation process. For example, in 1987 a French breed-and-burn reactor, the Superphénix, shut down due to a massive sodium leakage in its storage drum. The cause of this leak could be derived from the existence of impurities within the sodium, like oxygen, hydrogen, carbon and nitrogen. Oxygen, said to be the most dangerous impurity, should remain under 10 parts per million (ppm) while within a reactor [18]. In other words, per kilogram or liter of sodium, the content of oxygen within it should be less than 10 milligrams.

Nuclear engineers have implemented a system of apparatuses for monitoring these impurities, all of which aim to maintain their detection limits within 2 ppm of oxygen, 4 ppm of carbon, and 1.6 ppm nitrogen [18]. In the case of an impurity exceeding these limits, there have been advancements in purification systems. The most commonly used purification system for sodium is the cold purification through the use of cold traps. Cold traps, through means of heat and mass transfer devices, continuously cycle through the sodium, precipitating out dangerous impurities that might accumulate [20].

In summary, the usage of liquid sodium as a coolant for TWRs is extremely advantageous. Neutron traveling and heat transfers are greatly benefitted when liquid sodium is enacted. Though implementing this coolant can be tricky with its high reactivity with water and oxygen, engineers have successfully been utilizing careful transportation and monitoring techniques to greatly reduce these negative risks.
Heat Transfer
Fission fragments released from plutonium and uranium have high amounts of kinetic energy, which is utilized through processes similar to PWRs. During a reaction, heat that is generated first passes through the fuel cladding and then into the sodium pool. The heated sodium is then cycled through a heat exchanger where the heat is transferred to water. The water, having a lower boiling point than sodium, quickly boils and generates steam which is used to turn a turbine. The mechanical energy put into turning the turbines is converted into electrical energy. A typical sodium reactor thermal system is shown in Figure 6.
FIGURE 6 [21]

The heat transfer cycle of a sodium-cooled reactor
It can be seen that this is similar to a PWR with its separate coolant loops, though with sodium as an intermediate coolant. Because of the high temperatures which the sodium can reach (550 °C compared to LWRs which reach 300 °C), the heat transfer efficiency is much greater than in a LWR [22]. The higher thermodynamic efficiency of this system will allow for more electrical energy to be produced per unit of heat released from fission, making it a more sustainable process for harnessing energy from nuclear fuels.
PROBLEMS AND UNCERTANTIES
A question that is often asked about TWRs is: if TWRs have such great promise and the idea has been around for over 50 years, why has one not been built yet? Currently, the TWR still has a few obstacles to get through, most notably the cladding design, usage of a sodium coolant, proper funding, government regulations, and water resource concerns.

It has been stated before that sodium-cooled fast reactors have a checkered past and there is still uncertainty whether sodium is the ideal candidate. Since implemented, consistency has yet to be seen in sodium-cooled fast reactors. There have been recent reactor closings, like the Monju reactor in Japan which suffered from two major sodium-related accidents in its history and was finally shut down in 2010. However, advancements are continuing to be made in monitoring and purification techniques [23].

Also, which will most likely be the largest obstacle to overcome, is the problem of obtaining enough economic support to finance TWRs. Since no TWR has ever been built, it is hard for investors to take such an expensive financial risk. Though TWRs would save uranium enrichment costs, their capital costs are still estimated at around $1000 per kilowatt more than LWRs [23]. Also, since TWRs are so innovative, there will be a delay until TWRs can be properly licensed and regulated. In fact, the Nuclear Regulatory Commission (NRC) would essentially have to write new regulations specifically geared towards the TWR to account for its new mechanisms. It may seem flattering, but to do so the NRC would have to secure all the necessary safety calculations that would go along with it. To bypass such a delay, TerraPower could explore foreign agreements, though many other countries are just as inexperienced [23].

Lastly, a major concern for the general nuclear power community is for an even more critical resource: water. It is estimated that every day between 10 and 20 million gallons of water are evaporated from reactors; that is not even including daily water intakes [23]. To some, it’s hard to support nuclear energy when there are other clean energies that use almost no water at all.


THE TRAVELING WAVE REACTOR AS A POTENTIAL SOLUTION TO THE ENERGY DILLEMA
Though there are many testing and design challenges that still need to be overcome, the advantages that the Traveling Wave Reactor could have in the energy field is great. Utilizing fuel more effectively is a practice that is crucial in trying to meet the world’s growing energy needs. Through the use of a more abundant isotope of uranium and more efficient thermodynamic systems, more fuel will be utilized and the resulting energy will be more effectively harnessed as electricity. In these ways, the Traveling Wave Reactor could drastically increase the amount of energy available on Earth. In an interview, John Gilleland made the following assertion: “We figured out that we could supply every person on earth with the U.S. level of per capita energy consumption for 1000 years if we can make the traveling wave reactor go” [11].

Not only is this approach to nuclear energy more sustainable in terms of its ability to support the world’s growing energy needs, it also offers reductions in waste and environmental pollution. By simplifying the nuclear fuel cycle through the elimination of enrichment plants, the main source of carbon dioxide emissions will be eliminated and the amount of radioactive waste will be reduced. Also, eliminating enrichment plants will increase public safety by making it more difficult to manufacture weapons grade uranium. Though there may be technological and financial hurdles that will need to be overcome, the Traveling Wave Reactor could play a very important role in the future of sustainable energy.


REFERENCES
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[2] F. Spellman, R. Bieber. (2011). The Science of Renewable Energy. Boca Raton, FL: CRC Press. (Print Book).

[3] “Outline History of Nuclear Energy”. (2010, October). World Nuclear Association. (Web Article). http://www.worldnuclear.org/info/Nuclear-Fuel-Cycle/Fuel-Recycling/Plutonium/

[4] R. Murray. Nuclear Energy. (2009). Burlington, MA: Elsevier. (Print Book).

[5]R. Reister. (2012, June). “Status of Light Water Reactor (LWR) Activities in the US”. U.S. Department of Energy. (PDF).

[6] (2014). World Nuclear Association. (Web Page). http://www.worldnuclear.org/



[7] R. Nave. Georgia State University. (Web article) http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/reactor.html#c3

[8] “Terra Power”. (2014). Terra Power, LLC. (Webpage). http://terrapower.com/

[9] Contemporary Physics Education Project (CPEP). (2003 November). “Nuclear Science—A Guide to the Nuclear Science Wall Chart”. (E-book).

[10] ENDF/B-VII.1. “ENDF Request #13702 (201)”. Nuclear Energy Data Center.

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[12] K. Weaver, J. Gilleland, C Ahlfeld, C. Whitmer, G. Zimmerman. (2010). “A Once-Through Fuel Cycle for Fast Reactors”. Journal of Engineering for Gas Turbines and Power. (Print Article). Vol.132.

[13] Robert Petroski, Jesse Cheatham, Pavel Hejzlar, et al. (2012 June 24-28). “Traveling Wave Reactor Core Design Using Massive Parallel Precomputation”. TerraPower LLC. (Print Article).

[14] TerraPower. (2011 May 2-5). “TWR-P”. (Print Paper). Proceedings of ICAAP 2011. Paper 11199.

[15] C. Ahlfeld, T. Burke, T. Ellis, et al. (2011 May 2-5). “Conceptual Design of a 500 MWe Traveling Wave Demonstration Reactor Plant.” Proceedings of ICAAP 2011. Paper 11199.

[16] R. Vitaly, E. Linnik, V. Tarasov, et al. (2011). “Traveling Wave Reactor and Condition of Existence of Nuclear Burning Soliton-Like Wave in Neutron-Multiplying Media”. Energies. (Print Article). ISSN 1996-1073.

[17] E. Greenspan. (2012). “A Phased Development of Breed-and-Burn Reactors for Enhanced Nuclear Energy Sustainability.” University of California Nuclear Engineering Department. (Print Article).

[18] U. M. Poplauskii, A. D. Efanov, F. A. Kozlov, et al. (2010). “Sodium as a Coolant for Fast Reactors”. Springer Science+Business Media, Inc. (Print Article).

[19] E. Khodarev. “Liquid Metal Fast Breeder Reactors.” International Atomic Energy Agency. (Print Article).

[20] V. V. Alekseev, et all. (2013). “Sodium Coolant Purification System for a Nuclear Power Station Equipped with a BN-1200 Reactor.” Nuclear Power Stations. (Print Article).

[21] Janne Wanellius. “Liquid Metal Coolants”. Royal Institute of Technology. (PDF).

[22] “What is a Nuclear Reactor?” European Nuclear Society. (Online Article). https://www.euronuclear.org/1-information/energy-uses.htm



[23] A. Makhijani. (2013). “Traveling Wave Reactors: Sodium-cooled Gold at the End of a Nuclear Rainbow?” Institute for Energy and Environmental Research. (Print Article).
ADDITIONAL SOURCES
R. W. Lamphere. (1956 December 15). “Fission Cross Sections of the Uranium Isotopes, 233, 234, 236, 238, for Fast Neutrons”. (Print Article).
D. G. Madland. (2006). “Total prompt energy release in the neutron-induced fission of 235U, 238U, and 239Pu. Science Direct. (Print Article).
ACKNOWLEDGEMENTS
At this time we would like to thank several people who have greatly aided us along the way. First, Rachel Rohr for giving us valuable insight from her previous experience. Also Nick and Jack Andes, for informing us on many interesting nuclear engineering topics, as well as helping us with comprehension. And finally, our writing instructor, Julianne McAdoo, for giving valuable comments when reviewing our paper.





University of Pittsburgh Swanson School of Engineering

4/03/14


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